Active development has been recently carried out for laser diode modules based on a high power laser diode (LD).
A conventional LD module primarily includes a LD and a lensed fiber. A lensed fiber is an optical fiber, one of end faces of which is shaped like a lens (hereinafter, a “lens section”). The lensed fiber converges laser light emitted from the LD onto a core section by means of a light-converging function of the lens section.
The conventional, lensed fiber-based LD module, however, allows light that is not converged by the lens section to be coupled into a cladding section as unwanted light or excess light. For example, in the case of a high power LD with an output power of about 10 W, about 0.5 W to 1 W of light may be coupled into the cladding section. Therefore, if the light coupled into the cladding section was absorbed by a coating section or a metal coating section formed in periphery of the optical fiber, the coating section or the metal coating section could be heated to high temperature and thereby damaged.
These problems can be prevented by configuring such that light traveling in the cladding section is coupled into a radiation mode before the light enters the region where the coating section or the metal coating section is formed, so that the light is radiated out of the optical fiber. Some of techniques that are available for causing the light traveling in the cladding section to be coupled into such a radiation mode are an optical fiber disclosed in Patent Literature 1 and an optical transmission fiber disclosed in Patent Literature 2.
The optical fiber disclosed in Patent Literature 1 includes an inner cladding which is made of a material with a low refractive index and an outer cladding which includes (i) small regions filled with air and (ii) scatterers. The small regions filled with air in the outer cladding are partially collapsed along a light-guiding direction. By means of the scatterers contained in the outer cladding, the optical fiber disclosed in Patent Literature 1 scatters light which has leaked into the outer cladding.
In contrast, the optical transmission fiber disclosed in Patent Literature 2 includes an outer cladding layer which is thinner in thickness and lower in refractive index than an inner cladding layer. With the refractive indices set to a suitable value, the optical transmission fiber refracts stray light propagating in the inner cladding layer and captures the stray light in the outer cladding layer. Then, with the thickness of the outer cladding layer being suitably selected, the optical transmission fiber diffuses the captured stray light into a coating.
Well-known technology capable of selectively coupling specific modes to each other is optical fiber gratings (hereinafter, simply “FGs”).
The FG is an optical fiber-type optical element in which perturbation in refractive index (hereinafter, “refractive index grating”) is formed in a light-guiding direction of an optical fiber.
The refractive index grating of the FG enables the selective coupling of specific modes.
Such FGs are manufactured by irradiating an optical fiber with ultraviolet light so as to change a refractive index of a core section by photorefractive effect. Photorefractive effect is a phenomenon in which, for example, a refractive index of silica glass (SiO2) to which germanium (Ge) is added as a dopant increases when ultraviolet light of a wavelength of about 240 nm is shone on the silica glass. The FG has an advantage that it requires no major changes in the basic structure of a typical optical fiber.
Most technology that is related to such conventional FGs involves refractive index gratings formed in a core section. Patent Literature 3 discloses a FG in which gratings are formed in a core section and a cladding section. In addition, Patent Literature 4 discloses a FG in which refractive index gratings are formed in parts of a cladding section which are close to a core section.